1. Field of The Invention
This invention relates to optical communications systems and, more particularly, a dynamic gain control system for optical amplifiers.
2. Description of Related Art
Optical communications systems are desired because of the wide bandwidths available for the information signal channels. Speed limitations of optical transmitters and receivers make it difficult to fully exploit this wide bandwidth while making use of a single wavelength. Therefore, large capacity optical transmission systems are in general combining high speed systems on a signal fiber by means of Wavelength Division Multiplexing (WDM) to fill the available bandwidth. In these WDM optical transmission systems in general, rare-earth doped fiber optical amplifiers (such as Erbium or Erbium-Ytterbium doped) are used to compensate for the fiber link and splitting losses. Optical amplifiers under specific conditions add unwanted noise to the system. In addition, rare earth-doped fiber amplifiers present certain challenges for some applications.
Gain transients in optical amplifiers are a major problem for wavelength division multiplexed (WDM) optical systems in which channels are added or dropped either due to network reconfiguration or failures. Adding channels can depress the power of the present channels below the receiver sensitivity. Dropping channels can give rise to error events in the surviving channels because the power of the surviving channels can surpass the thresholds for non-linear effects, such as Stimulated Brillouin Scattering (SBS) which causes unwanted reflections and noise and Four Wave Mixing (FWM) which adds crosstalk between the channels. The error bursts in the surviving or present channels as a result of these power transients are unacceptable to service providers.
Furthermore, the gain for Erbium-doped fiber amplifiers is not uniform over the optical wavelength because the gain of the Erbium-doped amplifier is inherently dependent upon the absorption and emission wavelength spectrum of the Erbium ions in the fiber. This becomes a significant problem in WDM systems where multiple wavelengths are to be amplified simultaneously. As such, there is a need for Erbium-doped fiber amplifiers (EDFAs) with relatively constant multi-channel gains independent of changing WDM input channel numbers, powers, wavelengths and modulation schemes and with low gain variations between the WDM channels. For Erbium-Doped Fibers (EDFs) there exist inversion levels which provide low multi-channel gain variations. The inversion levels refer to the number of inverted or excited Erbium ions/the total number of Erbium ions in the fiber. For the ideal version of an EDFA, the gain variation between the WDM channels is zero. In practice, an EDFA having low multi-channel gain variation can still produce small channel gain variations between the channels (for example, 1 dB) which can be reduced using a passive optical filter, thereby producing a relatively flat and constant EDFA multi-channel gain v. wavelength spectrum within the EDFA's operating wavelength range.
In A. K. Srivastava et al., "Fast Gain Control in an Erbium-Doped Fiber Amplifier," Technical Digest of the Optical Amplifiers and their Applications 1996 Topical Meeting, OAA'96, Postdeadline paper PDP4, an output power control system measures the output power from a channel and drives the pump laser with a feedback loop to keep the output power of the channel relatively constant. This system does not provide real gain control because gain control is provided only for constant input power. The disadvantage of this solution is that it will not work when the channel that is used to measure the output power is dropped itself. Additionally, as mentioned above, this system will not maintain the multi-channel gain and the non-variation of the multi-channel gain spectrum when the input power of the measured channel is changed. In K. Motoshima et al., "Dynamic Compensation of Transient Gain Saturation in Erbium-Doped Fiber Amplifiers by Pump Feedback Control," Optical Fiber Communication Conference (OFC'93), San Jose, Calif., 1993, pp. 40-42, Paper Tu15, a system is described involving an extra probe signal of which the wavelength is located within the EDFA gain wavelength region but outside the possible signal wavelengths to measure and stabilize the gain. Both the input power and the output power of the probe signal are then measured to determine the actual gain. By using a feedback circuit which drives the pump laser diode current and therefore its power, the system is described as making the gain relatively non-varying. The disadvantage of this technique is that an extra DFB laser is needed in every EDFA to provide the probe signal as well as sharp wavelength multiplexers to add and extract the probe signal, thereby increasing the EDFA production costs.
J. Massicott et al., "Asymmetric Control Laser Cavity Design for Low Noise Operation of an All-Optical Gain Controlled Erbium-Doped Fiber Amplifier," Technical Digest of the Optical Amplifiers and their Applications, 1996 Topical Meeting, OAA'96, Paper FB2, pp. 77-80, reports a solution involving optical gain clamping. The use of optical gain clamping, however, requires at least two extra single wavelength optical reflectors. Furthermore, the additional loss at the signal wavelengths caused by the filters and/or reflectors needed for the gain clamping control laser increases the obtainable noise figure and decreases the obtainable output power of the amplifier given the available pump power.
Therefore, an EDFA is needed to overcome the disadvantages of the current systems and with relatively constant multi-channel gains independent of changing WDM input channel numbers, powers, wavelengths and modulation schemes.